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My fellow forum members, thank you for reading! I have a pair of KSP400s that I have painstakingly re-built and kept working- hum free- since purchasing in 1998! I have had to replace OP amps, power transistors, bridge rectifiers, big CAPS, little CAPS, positive and negative voltage regulators, just to name a few parts to accomplish this. For my trouble, I have 20 year old speakers that work as new. Since Klipsch long ago stopped their factory support for KSP400s in favor of third-party repair shops (no guarantee a "fixed" board will out-live the original).... Here's my end game: I recently retired and an electrical engineer by degree. I am seeking a "working fine business" sub-woofer amplifier board P0503, an amp board with a "bad hum or any other issue", or one that is just plain "dead" and may still be in your junk pile! Maybe you gave-up on these amps and performed an implant of a non-OEM amp into the KSP400 cabinet or you just deleted the built-in amp completely. Whatever may have caused you to have one amp you would part with, I need it!. If yours blows fuses immediately, hums and buzzes and wakes the neighbors, or plays great but is just surplus to your needs....I want it! I will pay fair price depending on degree of function. A good one will become a parts donor or my benchmark to establish test criteria for the "new & improved. Prefer a PA that has "issues" to minimize my out-of-pocket costs and may show me issues with these amps that I have just not encountered (yet). Do NOT feel your amp or control board is "too bad to help me" and the cause. My goal is to re-engineer a replacement amp that can hopefully occupy the stock board size, mounting points, etc.. I will be starting with the OEM transformer and beyond that, I see most all the rest going. My first board will initially be all point-to-point wiring until I get the amp Klipsch wished they built- one that will last, be simple but a brute as theirs was when working, with trouble-free longevity. It would be great to have a spare CONTROL PANEL as well, but I have had very few issues in the control panel to-date and that will be my next undertaking when I get the amp right. I may have to get a control panel-- depending on the PA board design changes-- to send proper control voltages, "auto on" control feature, and the like. Right now, I would settle for the PA board if a control panel is not forthcoming. Once finished with a fully functional PA board that I have tested and am 100% confident will perform well against current state-of-the-art amps, I will breadboard and then make a new circuit board to compliment my amp. My goals are that my board will be plug and play (fit existing space, heat sink, use existing control cable, very possibly use your existing control panel and will have equal or better RMS output and response characteristics than the OEM. I am a tad skeptical the existing control panel will be left untouched as I intend to use a 3 prong plug again and solve ground loop issues. Worst case is that oem control panel will have modifications that anyone handy with a solder pencil can facilitate. This entire undertaking MUST be an improvement without sacrifice in any respect to original. This is a totally NON-PROFIT under-taking!! I will sell just circuit boards alone at cost with a parts list and sources for all parts, and a detailed schematic. Any part I use WILL be readily available via Ebay or MOUSER electronics (worst case). Decent single-purpose amps such as these are easy and relatively simple with minimal parts. I envision the main PS filter electrolytics to be most expensive component on my board at under $10 (assumes you have oem transformer from your old board). Will try to allow using as many Klipsch oem board parts as possible for those willing to swap from their P0503 boards (as long as design is not compromised). I love a challenge, retirement leaves me too much time on my hands, and I have been more of an electronics "tech" than an electrical engineer since age 8; when I became an advanced class ham radio operator and built the first of 60+ Heathkits--- until Heathkit went belly-up. I worked 280 countries with equipment that I made "homebrew". I built many 2000 watt linear amps from scratch as my license allowed over the years, but most of those 280 countries were gotten on my QRP (under 1 watt) gear. Sorry, don't want to digress! Just to illustrate I am not new to making my own solutions and feeding off the results. The number of us who enjoy working with electronics from scratch to a fully functioning "finished product"-is dwindling. Help me fill my days, have fun doing it, and providing back to other Klipsch aficionados. I see no reason why this undertaking should take more than *maybe* 6 months-start to finish. I reside in Lafayette, IN. so you probably know what great engineering school is in my backyard to help with circuit board etching, modeling services, expedited non-destructive (I hope) testing, etc.! If anyone has the ability to DONATE a PA board or control panel, I will see that you get first consideration on any new solutions for PAYING IT FORWARD. Thanks all... for reading and any help, in advance. Bill P.S. prior owner of pair of K-horns, series II Cornwalls, and a few Synergy series surrounds, etc. My kids all have systems to kill for as I wanted to give while still alive to see their awe! Oldest son even got my Macintosh amps to go with the K-horns. Apartment life on fixed income is a *****...